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Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Protection & Fault Analysis/Discrimination & Coordination

lesson · 8min · Lesson 21 of 29

What is discrimination and why it matters

Course syllabusCourse overview
01Overcurrent Protection Devices
  1. ReadRewirable fuses vs cartridge fuses
  2. ReadMCB operating characteristics: B, C, D curves
  3. ReadRCBO: combined RCD + MCB operation
  4. ReadAFDD: arc fault detection
  5. quizDevice selection quiz
02Fault Loop Impedance
  1. ReadWhat is earth fault loop impedance?
  2. ReadZe: external impedance measurement
  3. ReadZs: total loop impedance calculations
  4. ReadDisconnection time requirements: Table 41.1
  5. exerciseZs calculation exercises
03RCDs and Residual Current
  1. ReadHow RCDs work: the core balance principle
  2. ReadRCD types: Type AC, A, F, B
  3. ReadRCD ratings: 10, 30, 100, 300mA
  4. ReadRCD testing and nuisance trips
  5. quizRCD selection quiz
04Prospective Fault Current
  1. ReadPFC: what it is and why it matters
  2. ReadCalculating PFC at the origin
  3. ReadPFC at distribution boards downstream
  4. ReadFault current rating of devices
  5. exercisePFC worked problems
05Discrimination & Coordination
  1. ReadWhat is discrimination and why it matters
  2. ReadCurrent discrimination
  3. ReadTime discrimination
  4. ReadEnergy discrimination for MCBs
  5. exerciseDiscrimination case study
06Testing for Protection
  1. ReadTesting Zs with loop testers
  2. ReadMeasuring PFC at the board
  3. ReadRCD trip time testing
  4. quizFinal assessment
Lesson · 8min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

What is discrimination and why it matters

Protection Fault Analysis · Lesson 21 · Advanced

AdvancedReview: professional review pending

Purpose

Explain selectivity (discrimination), backup protection, and coordination as different evidence-based design claims.

Before you beginProtective-device curves · Prospective fault current

Learning objectives

  • Define selectivity
  • Separate selectivity from backup protection
  • Identify the affected fault range
  • Use manufacturer evidence

Selectivity aims to disconnect the protective device nearest a fault while upstream healthy circuits remain supplied. It is a bounded claim for stated devices, currents, fault types, and conditions, not a property inferred from labels alone.

SELECTIVITY AND COORDINATIONSELECTIVITY AND COORDINATIONSOURCETRIPfault / loadcurves / time / I-squared-t / paired data

Core theory

Total selectivity means the downstream device operates alone throughout the relevant fault-current range; partial selectivity is demonstrated only up to a stated current.

Backup or cascading permits an upstream device to help a downstream device interrupt a fault above the downstream device's independent capability. That can improve short-circuit capability without guaranteeing selectivity.

Coordination checks overload and short-circuit curves, let-through energy, breaking/withstand ratings, RCD time/current behavior, conductor protection, assembly data, supply configuration, and the consequences of losing upstream circuits.

Terms, symbols, and units
TermMeaningSymbolUnit
SelectivityCoordination intended to isolate only the protective device nearest the faultNot applicableNot applicable
Partial selectivitySelectivity demonstrated only up to a declared currentNot applicableNot applicable
Backup protectionUpstream device assistance that raises a downstream combination's fault capabilityNot applicableNot applicable
Worked exampleA 16 A MCB is downstream of a 63 A MCB. Does the rating ratio prove selectivity?

Assumptions: Exact product data is not supplied.

  1. Overlap: Compare both time-current envelopes over overload and short-circuit regions.
  2. Energy: Check manufacturer selectivity and let-through tables for the exact pair.
  3. Limit: Compare any declared selectivity limit with maximum fault current at the downstream point.

No; the ratio is only a screening clue. Exact coordinated-device evidence is required.

Reasonableness check: Both instantaneous regions can overlap even when rated currents differ.

Common mistakes
  • Calling backup protection discrimination
  • Claiming total selectivity from rating ratio
  • Ignoring maximum fault current

Where this appears in practice

Selectivity limits disruption in distribution boards, process supplies, life-safety services, and domestic circuit division.

SafetyA coordination error can expose equipment to energy above its withstand or disconnect a wider area during a fault. Do not create faults to demonstrate selectivity.
Local code checkConfirm the current BS 7671 edition, exact device and assembly data, manufacturer selectivity/backup tables, required disconnection times, test-instrument ratings, safe system of work, and certificate schedule. A generic rating ratio or one field reading is not proof of coordination or compliance.

Knowledge check

Can a manufacturer-declared backup combination be assumed selective?

No. Backup capability and selectivity are separate declarations.

Answer: No. Backup capability and selectivity are separate declarations.

Practical exercise

Build an evidence checklist for an upstream/downstream pair without approving a real installation.

Summary

  • Selectivity is a bounded claim
  • Backup is a different function
  • Exact paired-device evidence governs

Sources and review

  • Guidance Note 6: Protection Against Overcurrent: IET; 10th edition, BS 7671:2018+A4:2026; United Kingdom
  • Inspection and Testing FAQs: IET; Current online guidance; United Kingdom
  • Electricity at Work Regulations 1989: HSR25: Health and Safety Executive; Third edition; Great Britain

Editorial review date: 2026-08-21. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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